How long are the times between dig attempts?

I really can't remember. I'm pretty sure its a range, but at least an hour. Its why I always just set the alert to tell me when I can dig again.

You can also add the Coda Caves Widget to your dashboard. It shows a gem that's either red (can't dig) or green (can dig) with a link to the caves. To add a widget to your dashboard go to personal->dashboard, then choose the widget tab across the top menu, or click here.